The present invention relates to digital communications, and more particularly to mobile wireless systems and methods.
Spread spectrum wireless communications utilize a radio frequency bandwidth greater than the minimum bandwidth required for the transmitted data rate, but many users may simultaneously occupy the bandwidth. Each of the users has a pseudo-random code for “spreading” information to encode it and for “despreading” (by correlation) received spread spectrum signals to recover the information. Such multiple access typically appears under the name of code division multiple access (CDMA). The pseudo-random code may be an orthogonal (Walsh) code, a pseudo-noise (PN) code, a Gold code, or combinations (modulo-2 additions) of such codes. After despreading the received signal at the correct time instant, the user recovers the corresponding information while other users' interfering signals appear noise-like; indeed, a single user can receive multiple independent channels of information through use of multiple spreading codes. For example, the interim standard IS-95 for such CDMA communications employs channels of 1.25 MHz bandwidth and a pseudo-random code pulse (chip) interval Tc of 0.8138 microsecond with a transmitted symbol (bit) lasting 64 chips. The recent wideband CDMA (WCDMA) proposal employs a 3.84 MHz bandwidth and the CDMA code length applied to each information symbol may vary from 4 chips to 512 chips. The UMTS (Universal Mobile Telecommunications System) approach UTRA (UMTS Terrestrial Radio Access) provides a spread spectrum cellular air interface with both FDD (frequency division duplex) and TDD (time division duplex) modes of operation. UTRA currently employs 10 ms duration frames partitioned into 15 time slots with each time slot consisting of 2560 chips (Tc=0.26 microsecond).
The CDMA code for each user is typically produced as the modulo-2 addition of a Walsh code with a pseudo-random code (two pseudo-random codes for QPSK modulation) to improve the noise-like nature of the resulting signal. A cellular system could employ IS-95 or WCDMA for the air interface between the base station and multiple mobile user stations.
A spread spectrum receiver synchronizes with the transmitter by code acquisition followed by code tracking. Code acquisition performs an initial search to bring the phase of the receiver's local code generator to within typically a half chip of the transmitter's, and code tracking maintains fine alignment of chip boundaries of the incoming and locally generated codes. Conventional code tracking utilizes a delay-lock loop (DLL) or a tau-dither loop (TDL), both of which are based on the well-known early-late gate principle.
The air interface leads to multipath reception from a single transmitter, and a RAKE receiver has individual demodulators (fingers) tracking separate paths and combines the finger results to improve signal-to-interference-plus-noise ratio (SINR). The combining may use a method such as the maximal ratio combining (MRC) in which the individual detected signals in the fingers are synchronized and weighted according to their signal strengths or SINRs and summed to provide the decoding statistic. That is, a RAKE receiver typically has a number of DLL or TDL code tracking loops together with control circuitry for assigning tracking units to the strongest received paths. Further, arrays of antennas allow for detection of and transmission with signal directionality by phasing the combined signals among the antennas the signals from or to a single user. FIGS. 2a-2d Illustrate functional blocks of various CDMA receivers and transmitters.
For FDD mode the physical synchronization channel appears in each of the 15 time slots of a frame and occupies 256 chips out of the 2560 chips of the time slot. Thus a base station transmitting in the synchronization channel a repeated primary synchronization code of pseudo-noise of length 256 chips modulated by a length 16 comma-free code (CFC) allows a mobile user to synchronize by first synchronizing to the 256-chip pseudo-random code to set slot timing and then using the cyclic shift uniqueness of a CFC to set frame timing. Further, decoding the CFC by the mobile user reveals the scrambling code used by the base station.
Antenna arrays for the base station (e.g., 2 to 16 antennas in a linear array) and possibly also for the mobile users (e.g. 2 antennas) can improve data rates or performance. For example, transmit adaptive array (TxAA) is a two-antenna diversity technique which adjusts antenna weights (relative phase and possibly also the power balance between the two antennas) at the base station to maximize SINR at the mobile user. TxAA can be used in the proposed 3GPP standard for a high speed downlink packet access (HSDPA) in WCDMA which transmits packets in a 2 ms (3 time slots) transmission time interval (TTI). Packets may be transmitted with either 16-QAM or QPSK modulation. Within a TTI the ratio between the transmitter power for the pilot symbol channel (CPICH) and the transmitter power for the high-speed downlink shared channel (HS-DSCH) does not change. This requirement of a constant power ratio during a TTI allows a mobile user to estimate the power ratio in the first time slot of the TTI which would then be valid for the remaining two time slots of the TTI. Indeed, due to the timing constraints at the mobile user for decoding the packet and reporting an ACK or NAK to the base station, there is insufficient time to make power ratio estimations in every time slot if each estimation takes one time slot. This implies a problem of changing antenna weights within a TTI for TxAA mode 1 and mode 2 which adjust weights at the base station based upon feedback (e.g., FDD) from the mobile user.